- 밥 & 식사 & 진지
진지 is more honorific.
You can use 식사 to your senior, peers, co-wokers, boss, people who are older than you.
- 영희씨, 언제 한번 식사 같이 해요.
- 철수씨, 식사 하세요.
- 사장님, 식사 하셨어요?
- 여러분, 식사하러 갑시다.
You can use 진지 to your grandparents and people who are grandparents' age.
- 할아버지, 진지 잡수세요. (=진지 드세요.)
- 할머니, 진지 잡수셨어요? (=진지 드셨어요?)
You can use 밥 to your friends and people who are younger than you.
- 철수야, 밥 먹으러 가자.
- 야, 밥 먹었어?
- 언제 밥 한번 먹자.
And, 밥 and 진지 means the food stuff(밥 and 반찬 or something). and 식사 means the action(To eat some food, or to have a meal).
[밥 or 진지] + [먹다 or 잡수시다 (To eat)]
[식사] + [하다 (To do)]
I think a difference should be make between those honorifics who are used to rise the position of the person who you are talking about and those who puts you (the person who speak) in an inferior position.